1. Field of the Invention
The present invention relates to a process for the synthesis of alkoxysilanes, particularly trialkoxysilanes such as trimethoxysilane. More particularly, the present invention relates to a vapor-phase process for reacting aminosilanes, alkoxyaminosilanes, aminocarbamatosilanes, alkoxycarbamatosilanes and alkoxyaminocarbamatosilanes with alcohols in the presence of a catalyst and under the appropriate reaction conditions to produce these trialkoxysilanes.
2. Prior Art
Trialkoxysilanes are used extensively in hydrosilation reactions, especially those directed at the manufacture of silane coupling agents. See, for example, Plueddemann, Silane Coupling Agents, Chapter 2, Plenum Press, New York 1982.
The most common reaction for the synthesis of these trialkoxysilanes is an alcoholysis reaction using trihalosilanes as a starting reactant. See R. J. H. Voorhoeve, Organohalosilanes: Precursors of Silicones, pp. 301-306, Elsevier, New York, 1967. This reaction is as follows: EQU HSiX.sub.3 +3ROH.fwdarw.HSi(OR).sub.3 +3HX (I)
where X is halogen, R is a monovalent alkyl radical containing up to about twenty carbon atoms and HSi(OR).sub.3 represents a trialkoxysilane such as trimethoxysilane.
A major deficiency of this prior art process is that the Si-H bond in the trialkoxysilane is destroyed and replaced with an SiOR bond in a side reaction to produce tetraalkoxysilanes, Si(OR).sub.4, and mixed alkoxyhalosilanes, X.sub.a Si(OR).sub.4-a where a is an integer greater than or equal to one and less than or equal to three. While some tetraalkoxysilanes, e.g., tetraethoxysilane may have some commercial utility, most are undesirable by-products. In addition, tetramethoxysilane is known to be quite toxic, Chemical Engineering News, Vol. 24, p. 1690 (1946), and its formation should be avoided.
As seen in reaction (I), this side reaction is very difficult to avoid because the HX molecule catalyzes the reaction of alcohol, ROH, with the Si-H bond on the trialkoxysilane. Because 3 moles of hydrogen halide are produced per mole of trialkoxysilane made, and because there is a stoichiometric excess of alcohol in the conventional batchwise liquid phase process, this reaction is particularly difficult to avoid.
Further, if a more sterically hindered alcohol is used in reaction (I) such as tert-butyl alcohol for example, the hydrogen halide reacts with the alcohol to give tert-butyl halides and water; the reaction of water with the resulting mixed halo(tert-butoxy)silanes leads to the formation of crosslinked siloxane products which even further reduce the efficiency of the alcoholysis to form trialkoxysilanes.
The prior art contains a number of attempts to alleviate or circumvent the problems associated with hydrogen halide solubility in the reaction mixture. (C. Eaborn, Organosilicon Compounds, pp. 289-294, Butterworths, London, 1960). These attempts include the use of hydrogen halide acceptors, e.g., amines, the use of solvents to reduce hydrogen halide solubility in the reaction medium, as well as the use of low pressure to pump off the hydrogen halide, as it is formed. (W. Gerard and K. Kilburn, J. Chem. Soc. (1956), p. 1536). None of these methods, however, has been very successful.
In current manufacturing practice, hydrogen halides evolved during the alcoholysis of halosilanes are treated as a waste product. Typically these hydrogen halides are scrubbed with water and the acidic solution is neutralized with base prior to final discharge. The recovery and reuse of chemically pure hydrogen halides, e.g., HCl, from the gaseous discharge of the halosilane alcoholysis for their use in the direct synthesis of trihalosilanes, e.g., trichlorosilane, is difficult and uneconomic because of the need to remove contaminants such as alcohols, solvents and other organic compounds down to insignificant levels.
In order to overcome the problems associated with the formation and removal of hydrogen halide from the conventional liquid phase alcoholysis reaction product, U.S. Pat. No. 4,395,564 to Kanner et al. discloses a new liquid phase process for the synthesis of trialkoxysilanes based on the alcoholysis of tris(dialkylamino)silanes, e.g., HSi[N(CH.sub.3).sub.2 ].sub.3, in the presence of CO.sub.2, dimethylammonium dimethylcarbamate (DI-CARB), protic acids or Lewis acids as catalyst.
This reaction is shown below in the following schematic reaction: ##STR1##
While the Kanner et al. patent avoids the problems of the prior art and helps to increase the efficiency of production of trialkoxysilanes such as HSi(OCH.sub.3).sub.3, the reaction conditions taught by that patent, e.g., temperature and molar ratio, still result in a number of inefficiencies in this liquid-phase process.
For example, production of tetraalkoxysilane by-product seems to be more in the order of 10 to 30 weight percent than the 3 to 7 weight percent initially predicted in the patent. It appears that the dialkylamine, which is released during the reaction of tris(dialkylamino)silane, is retained in the reaction mixture as a by-product thus enhancing the loss of the Si--H bond on trialkoxysilane and aiding the formation of Si(OCH.sub.3).sub.4. In addition, the Kanner patent also results in a significant amount, i.e., up to 11 percent, of by-products such as hydrido(dialkylamino) alkoxysilanes, e.g., HSi[N(CH.sub.3).sub.2 ].sub.x (OCH.sub.3).sub.3-x where x equals 1 or 2 and dialkylamino-alkoxysilanes, e.g., (CH.sub.3 O).sub.x Si[N(CH.sub.3).sub.2 ].sub.4-x where x equals 1 to 3 which by-products persist in the reaction mixture even following the complete addition of 3 moles of alcohol to the DI-CARB plus tris(dialkylamino)silane mixture taught in the patent. If the crude reaction mixture is left in contact with dialkylamine by-product for at least a day, the concentration of these by-products rises even further, while conversely the concentration of trialkoxysilane decreases.
Thus there is no teaching in the art which recognizes that the retention of dialkylamine in the product mixture can cause an increase in the formation of tetraalkoxysilane or which recognizes the problem of redistribution of alkoxy and dialkylamino moieties on silicon leading to the persistence of dialkylaminoalkoxysilanes in the reaction product. Thus, there is a need in the art for a vapor-phase reaction of alcohol with various aminosilanes and carbamatosilanes to more efficiently produce alkoxysilanes, more particularly trialkoxysilanes, and reduce the level of undesirable by-products.